Expansion molding is one of important methods for molding and/or processing a polypropylene-based resin. Various kinds of molded articles obtained by extrusion foaming or injection foam molding are widely and effectively used for various purposes, taking advantage of excellent properties of them, such as heat insulating properties, sound insulating properties, cushioning properties, energy absorbing properties and the like. Especially in recent years, in view of environmental concerns, weight reduction of materials and reduction of environmental burdens become important problems of technical development. The technical area in which a molded foam is used tends to become wide, and demand for a resin with high expandability becomes high.
A commonly-used polypropylene-based resin has low melt tension, which an important index of foaming properties, because the molecules of this resin have linear structure and the molecular weight of this resin is not so high. For this reason, this resin is not suitable for expansion molding. In order to compensate for this drawback, various techniques have been developed in the past (for example, see PATENT LITERATURES 1 to 6).
For example, PATENT LITERATURE 1 discloses a technique in which long chain branches are introduced into polypropylene by high-energy ionization radiation to increase its melt tension. In general, the term “long chain branch” used herein means a branched structure with branches of molecular chains each having a main chain with several tens or more of carbon atoms (several hundreds or more of the molecular weight). In contrast, other kinds of branched structures, for example, a branched structure obtained by copolymerization of a-olefins each having a carbon number around 4 to 20 (each of which is frequently used as a comonomer), usually have no great effect on melt tension and the like. Such a branched structure is referred to as a “short chain branch” to distinguish it from the above-mentioned “long chain branch”. Similarly, as methods for introducing long chain branches into a polypropylene resin using an organic peroxide, many techniques are disclose in PATENT LITERATURE 2, PATENT LITERATURE 3, PATENT LITERATURE 4 and the like. In a technique frequently carried out by a person skilled in the art, a polypropylene having produced therein long chain branches by the above-mentioned methods is blended with another polypropylene-based resin. This blending is carried out considering the fact that in general, such a polypropylene is expensive because of particularity of the production method and in view of improvement in fluidity during various kinds of moldings and improvement in physical properties and appearance of molded articles. For example, PATENT LITERATURE 5 discloses a technique in which a composition comprising a polypropylene resin having long chain branches produced using electron beam radiation and a normal homopolypropylene or propylene-ethylene copolymer having no long chain branch gives a polypropylene-based resin foam which is inexpensive and excellent in heat resistance, heat insulating properties and heat insulating properties. PATENT LITERATURE 6 discloses a technique in which a polyolefin-based resin composition which is excellent in extrusion foamability and gives an extruded foam with improved brittleness is obtained by combination of a high-melt tension polypropylene with a natural logarithm of the melt tension and that of the melt flow rate satisfying a specific relationship and a low-melt tension polypropylene-based resin with a melting point 3 to 30° C. lower than that of the high-melt tension polypropylene.
However, in the technique in which long chain branches are introduced into polypropylene by high-energy ionization radiation or the use of an organic peroxide, there are some problems. In the former, there are problems of high production cost, yellowing, change in physical properties over time and the like. In the latter, there are problems of contamination with a decomposition product derived from the organic peroxide, odor, yellowing, safety during the production and the like. For this reason, there are many conventional techniques for increasing melt tension of polypropylene by another method. As one of such techniques, a technique for increasing melt tension by broadening the molecular weight distribution is disclosed.
For example, PATENT LITERATURE 7 discloses a method for producing polypropylene with high melt tension, high stiffness and excellent moldability by 2-step polymerization. In this method, a polypropylene with an intrinsic viscosity [η] of 0.5 to 3.0 dl/g is produced in the first step polymerization in an amount of 50 to 85 wt. % relative to the total amount of the final polymerization product. Then, a polypropylene with an intrinsic viscosity [η] of 9 dl/g or more is produced in the second step polymerization, in an amount of 50 to 15 wt. % relative to the total amount of the final polymerization product. As a result, a crystalline polypropylene with an intrinsic viscosity [η] of 2 to 6 dl/g, a melt flow rate (MFR) of 0.01 to 5 g/10 minutes and an isotactic pentad fraction of 0.940 or more, as a whole polypropylene, is obtained
The polypropylene-based resin composition obtained by the above-mentioned method has broad molecular weight distribution. However, this composition has problems in that it has poor moldability and gives a molded article with poor appearance. Based on this fact, PATENT LITERATURE 8 discloses that a polypropylene-based resin composition having a melt flow rate of 0.01 to 5 g/10 minutes, an intrinsic viscosity [η] measured at 135° C. in decalin of 8 to 13 dl/g, a gel number of 3000/450 cm2 or less, molecular weight distribution Mw/Mn of 6 to 20 and Mz/Mw of 3.5 or more can solve these problems since this composition has high melt tension, excellent moldability and high stiffness and makes it possible to produce a molded article with good appearance.
PATENT LITERATURE 9 discloses that a polypropylene with a specific value of biaxial elongational viscosity is a technique for obtaining a foam with high ratio of closed pores. However, close study of this document shows that this document contains the following 2 descriptions.
“It is noted that in the method disclosed in WO 91/13933 for producing the above-mentioned “specific propylene polymer resin comprising a major moiety of a linear propylene polymer and a minor moiety of side chains highly branched from the linear propylene polymer”, the side chains highly branched from the linear propylene polymer are formed using electron beams or radiation. Therefore, it is believed that during the linking of side chains to the main chain (i.e., branching), a cleavage of the main chain is likely to occur, so that the viscosity of the resin as a whole cannot be raised and, therefore, it is impossible for the resin to have an ultrahigh molecular weight which contributes to the high biaxial extensional viscosity of the resin to be used for producing the foam of the present invention.”
“The preparation of a propylene polymer resin to be used as the base resin for producing the foam of the present invention can be conducted by, for example, a two-stage polymerization method in which the polymerization reaction is conducted in the presence of Ziegler-Natta catalyst in a polymerization vessel having a fixed bed provided with a stirrer (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 4-226109 corresponding to EP 0 463 406 A2).”
From these descriptions, the essence of this technique is regarded as being based on broadening of the molecular weight distribution with ultrahigh molecular weight components. Close study of Examples disclosed in the above-mentioned PATENT LITERATURES 7 to 9 shows that each of the ultrahigh molecular weight components is constructed with polypropylene.
In these techniques using ultrahigh molecular weight components, the effect for improving melt tension is limited. PATENT LITERATURE 10 mentions that a polypropylene-based resin composition satisfying some specific physical properties can improve the melt tension of a polypropylene-based resin with no deterioration of its good properties, such as mechanical properties and chemical resistance, and achieve uniform and fine cells, the composition comprising:
50 to 90 wt. %, relative to the total weight of the composition, of a propylene homopolymer or propylene copolymer with a content of an olefin other than propylene of 1 wt. % or less having a melt flow rate of 10 to 1000 g/10 minutes, and
10 to 50 wt. %, relative to the total weight of the composition, of a propylene copolymer with a content of an olefin other than propylene of 1 to 15 wt. % having a weight-average molecular weight of 500000 to 10000000.
However, in the technique for improving melt tension using ultrahigh molecular weight propylene homopolymer or copolymer or a technique in which, as disclosed in PATENT LITERATURE 11, melt tension is improved by finely dispersing an ultrahigh molecular weight ethylene-α-olefin copolymer in another olefin (co)polymer, there is a problem in that the resultant melt tension is generally lower than that of a polypropylene having long chain branches. Also in the field of expansion molding with high expansion ratio or blow molding for producing large molded articles (which requires especially high melt tension), a polypropylene having long chain branches tends to be widely used. Problems of such a polypropylene are described above.
In recent years, there are proposals of methods with copolymerization of a macromer using a metallocene catalyst (hereinbelow, such a method is referred to as a “macromer copolymerization method”). For example, there is a proposal of a method (macromer copolymerization method) in which a propylene macromer having a terminal vinyl group is produced in the first polymerization step (step of macromer synthesis) using a specific complex under specific polymerization conditions and the resultant product is subjected to copolymerization with propylene in the second polymerization step (step of copolymerization of macromer) using a specific catalyst under specific polymerization conditions, the method giving a product with no higher crosslinking, no loss of chemical stability as a natural property of polypropylene, excellent in recyclability and no concern of gel generation in return for improvement of melt tension (see PATENT LITERATURES 12 and 13).
However, in this method, in order to effectively obtain the terminal vinyl group (which must be present in a macromer) in the former step, the polymerization must be must be conducted using a specific complex under relatively high temperature and low pressure. For this reason, the molecular weight and stereoregularity of the resultant macromer become disadvantageously low.
In contrast to the above-mentioned multi-step polymerization method, there is a proposal of a method for single-step polymerization (in situ macromer-producing copolymerization method) in which the step of macromer synthesis and step of macromer copolymerization are simultaneously conducted using a specific complex (see PATENT LITERATURE 14). However, in this method, the amount of each of the macromer and copolymerized macromer is not satisfactory, and the level of improvement of melt tension is not satisfactory.
By the techniques disclosed in PATENT LITERATURES 15 and 16, various problems in conventional techniques related to macromer copolymerization methods can be solved and a polypropylene having long chain branches with extremely high melt tension and good elongation viscosity can be obtained. Further, PATENT LITERATURES 17 and 18 propose a method in which a foamed sheet can be obtained using these techniques.
By blending the polypropylene-based resin having long chain branches obtained by the macromer copolymerization method with another polypropylene-based resin with other specific properties, various kinds of resin compositions suitable for various purposes can be obtained. These are disclosed in PATENT LITERATURES 19 to 27.
Especially, PATENT LITERATURE 27 discloses a polypropylene-based molded foam obtained by subjecting, to injection foam molding, a resin composition comprising (a) the polypropylene having long chain branches produced by the macromer copolymerization method of PATENT LITERATURE 15 or 16, (b) a polypropylene-based resin having a weight-average molecular weight of 50000 to 800000 and (c) a blowing agent.
As described above, there are many proposals of molded articles or compositions using a polypropylene-based resin having long chain branches, the molded articles or compositions showing excellent properties by the use of various molding methods. However, according to the present inventors, it is becoming apparent that some inherent, practical problems are still present in these resins. For example, in the extrusion foaming of a polypropylene-based resin having long chain branches to a foamed sheet as described in PATENT LITERATURE 18, there are actually some problems to be solved. For example, poor fluidity of this resin in an extruder frequently causes high load so that it is difficult to increase the extrusion rate. Further, insufficient extensibility of this resin during the withdrawal brings insufficient appearance of the resultant foamed sheet.